The present invention relates to water-tube boilers such as once-through boilers, natural circulation water-tube boilers and forced circulation water-tube boilers.
The water-tube boiler is a boiler the body of which is made up by water tubes. The body arrangement of such a water-tube boiler is, for example, that a plurality of water tubes are arranged into an annular shape. In the water-tube boiler of this form, a cylindrical space surrounded by the annular water tube array is used as a combustion chamber. In such a water-tube boiler, heat transfer primarily by radiation is performed within the combustion chamber, and then heat transfer primarily by convection is done in the downstream of the combustion chamber.
In recent years, such water-tube boilers are also desired to be further reduced in NO.sub.x and CO. The reduction in NO.sub.x, as it stands now, is implemented by fitting low-NO.sub.x burners or exhaust-gas re-circulation equipment to the existing boiler bodies. The reduction in CO is implemented by adjusting the state of combustion of the combustion equipment.